Various sports are widely loved and sports instruments specifically adapted to each sport have been used. Various industrial materials have been developed and new materials have been applied to the various sports instruments.
For example, in the field of tennis rackets, recently, there is a trend that larger rackets or rackets with larger frames than before, especially those made of a material which is light but yet has a sufficient strength and rigidity, are increasingly used.
The frames of conventional ordinary tennis rackets are made of wood, fiber-reinforced plastics (FRP) such as glass fiber-reinforced plastics and carbon fiber-reinforced plastics, or metals such as aluminum alloy. Recently, the percentage of those made of plastics, especially fiber-reinforced plastics (FRP) was sharply increased because of the developments in the molding technique, ease of production and due to their good reputation among tennis players.
Although the above-described materials used in the conventional rackets intrinsically have relatively good vibration-damping property, it is not sufficient for sharply and effectively damping the impact and vibration generated when a ball is hit.
Under these circumstances, recently, a large number of people of a wide range of ages have started playing tennis as a light sport. With this sharp increase in the population of the tennis players, number of tennis players suffering from disorders such as "tennis elbow" which is a disorder of elbow is also sharply increased.
It is thought that the disorder called tennis elbow is caused by the impact and vibration generated when a ball is hit with the gut face of a racket, transmitted through the racket frame to the elbow of the player. Especially, when a beginner or a middle class player who is not very good at playing tennis and who cannot properly hit the ball with the sweet spot continues to play tennis with its accompanying unnatural swinging a racket with a large frame made of a light material (Sweet spot is the central portion of the gut face. If the racket hits a ball with a portion other than the sweet spot, the impact and vibration generated thereby are transmitted to the elbow).
Further, even if the tennis elbow is not caused, the impact and vibration generated by racket when hitting a ball with a portion other than the sweet spot is transmitted to the hand, arm or elbow of the player and prevent the player from enjoying a comfortable play under sharp and proper hitting and playing feelings. Further, with such a hit force is not properly and effectively transmitted to the ball due to the generation of the impact and vibration, so that powerful and high level techniques cannot be performed. As a result, the fun of playing tennis is reduced.
Thus, the lighter the impact or the vibration from the racket when hitting a ball, the better. Further, if the impact or vibration is light, even if a portion somewhat outside the sweet spot hits the ball, the player feels as if the ball was hit with the sweet spot, so that even beginners can comfortably play tennis with comfortable hitting feelings.
On the other hand, for the purpose of reducing the impact and vibration when hitting a ball, a "stabilizer" has been proposed and is commercially available. The "stabilizer" is a molded article of rubber or soft synthetic resin, and is inserted between adjacent guts or is pressingly attached to the gut face. Although the "stabilizer" is effective for reducing the vibration of the gut per se, it does not function to effectively dampen the vibration transmitted from the gut face to the body of the player via the frame.
As can be seen from the above description, needless to say, it is very important that the frame structure per se effectively damp the impact or vibration transmitted from the gut face to the body of the player through the frame when hitting a ball, and realization of such a racket has been strongly desired with the proviso that the frame structure does not bring about a problem such that the overall weight of the racket is too heavy or the strength of the racket is too low.
Regarding other sports, in most sports in which a sports instrument is handled with the body such as arms or legs, there is a problem caused by the transmission of the impact or the vibration generated by the playing.
For example, the problem that the impact or vibration caused by hitting a ball gives undesirable results also resides in playing golf. Generally, if a ball is hit with the sweet spot of the head portion of a golf club, the ball gains the maximum initial velocity and the flying direction of the ball is also stabilized. On the other hand, if a ball is hit with a portion outside the sweet spot, the club head is rotated about the center of gravity thereof, so that the initial velocity of the ball is decreased and so the flying distance of the ball is decreased accordingly. Further, the direction of the flying out of the ball is shifted and so the ball may fly in an undesirable direction.
To improve the flying distance and flying direction of the ball, several proposals have been made. That is, it has been proposed to adjust the weight distribution of the head portion of a golf club so as to adjust the position of the center of gravity of the head and to increase the moment of inertia of the head (Japanese Utility Publication (Kokoku) No. 53-288). It was also proposed to change the horizontal and vertical lengths of the hitting area of the head portion of a golf club (Japanese Laid Open Utility Model Application (Kokai) Nos. 61-165762 and 63-192474). However, these proposals do not solve the problem of the impact or vibration caused by hitting a ball and do not solve the problem of the uncomfortable palsy feeling and accumulation of fatigue in the wrists, arms and elbows caused by the transmission of the impact and vibration generated when hitting a ball to the player.
Thus, a golf club shaft which has a function to effectively dampen the impact and vibration has been demanded.
To effectively prevent the transmission of impact and vibration to the person handling a sports instrument through the sports instrument, or to effectively damp an external impact and vibration by the properties of the sports instrument is desired in using other sports instruments than tennis rackets and golf clubs. Examples of the such sports instruments include rackets for other than tennis such as for squash, badminton and the like, skis, stocks for skiing, baseball bats, sticks for hockey, ice hockey, gate ball and the like, and bows and arrows for archery, Japanese archery and the like.